Publications and other reference materials referred to herein are numerically referenced in the following text and respectively grouped in the appended Bibliography which immediately precedes the claims.
Surface plasmon resonance (SPR) sensing devices have seen a tremendous awareness during the last decade both from a fundamental point of view and as highly sensitive devices for optical sensing of small biological or chemical entities in gases and liquids. There are two main types of surface plasmons (SPs): extended and localized. The extended SP is considered as more classical since it has been known for longer time. It can be described as a longitudinal electromagnetic wave (EM) in a two dimensional electron gas that exists on the surface of metals. Localized SPs on the other hand have become more familiar only during the last two decades. They are excited in metallic structures with dimensions less than half the wavelength of the exciting EM wave. In both cases the incident EM field has to be polarized in the plane of incidence (transverse magnetic (TM) polarization).
Biosensing is a strongly active field of research [1] due to the need for high sensitivity, rapid, specific and reliable biosensors both for environmental control and for medical diagnosis. Optical biosensors in particular are important for their simplicity, compactness, their potential to allow remote sensing and imaging to read many sensor chips in parallel [2]. Nylander and Liedberg [3] demonstrated high sensitivity sensing in the early 80's using surface plasmon resonance (SPR) phenomenon. Since then, SPR phenomenon based sensing has been receiving continuously growing attention [4]]. In order to improve on the already successful panoply of optical biosensors, nano-photonic structures composed of periodic metallic openings with dimensions less than the wavelength can be used Within the periodic structures, quite unexpected phenomena emerge related to the existence of localized SPR (LSPR) and extended SPR (ESPR) excitations. One of the peculiar phenomena is the enhanced optical transmission (EOT), in which transmittance through such nano openings, exceeds the relative area ratio (geometric limit) as predicted originally by Ebbesen et al. [5]. This result was mismatched with Bethe's aperture theory [6], which predicts negligible transmission through a single small hole in a thin metal film. The phenomenon of enhanced optical transmission is also termed extraordinary optical transmission. EOT causes large sensitivity because it involves two intertwined processes: (1) transmission through the nanoapertures and (2) scattering of evanescent waves by the apertures. Lee et al. [7,8] demonstrated a comparative study which verifies higher sensitivity of nanoslits over nanoholes based structures and therefore nanoslits are better candidates for sensing designs. One of the applications of EOT phenomenon is sensing in a water environment.
By attaching molecules to the nanoslits structure, the transmission is modified significantly and the overall transmission resonance undergoes a large shift in wavelength due to surface plasmons and cavity modes [9,10]. There are several conditions for surface plasmon excitation, which depend on parameters such as angle of incidence, grating geometry, wavelength, and the dielectric constants of the metal and dielectrics below and above the grating. The resonance is observed in terms of a sharp dip in reflection or sharp peak in transmission in the output optical signal at either the resonance angle (angular interrogation) or the resonance wavelength (spectral interrogation). Any change in refractive index near the interface causes a change in the value of the resonance location. This means that the resonance wavelength or incidence angle is sensitive to refractive index variations, a fact that makes this structure a potential sensor. In addition, periodically structured metallic films constituted of sub-wavelength apertures based on SPR phenomenon are potential sensors for a variety of applications including water and food quality control. It remains unclear however, whether this type of sensor is able to detect large biological or biochemical entities with dimensions larger than the slit width. In the food industry in general and in the water industry in particular, the quality of the product is evaluated by chemical (or microbiological) time dependent analysis. These processes include techniques such as chromatography, spectrometry, electrophoresis, etc., which enable recognizing water pollutants of small size and of small amounts. SPR based optical biosensors have gained attention due to their speed of detection, high specificity, high sensitivity and the possibility of on-line real-time analysis [11]. Since the extraordinary transmission of subwavelength grating based SPR sensors is a result of SPR, they are expected to have good surface sensitivity similar to conventional SPR sensors [8,12]. Subwavelength grating based SPR devices show sensitivity of about 400 nm/RIU [10,13] when designed to operate in the visible region of the spectrum.
In the visible range of the spectrum the enhanced local field extends only to a range of about 50-100 nm from the slits; hence the sensor will not sense the whole volume of a bacteria cell for example. This problem is well known in the traditional prism coupled SPR sensors even though extended SP waves have a relatively larger propagation length along the surface of the order of 10 μm [14]. Long range SPR configurations have been proposed [15] to resolve this problem. Sensors based on EOT, strongly depend on the detailed structure of the nanomaterial. It is well known that the aperture size and type of the metallic nanostructures have different contributions in the extraordinary transmission [16,17]. However, according to Lee et al. [8], the width of nanoslits does not affect the sensitivity of the sensor. The present inventor has proposed a methodology for increasing the sensitivity of resonant based structures by maximizing the electromagnetic interaction of the evanescent field with the analyte [18]. This methodology was also confirmed numerically to be valid for SPR sensors [19].
It is a purpose of the present invention to provide SPR sensors that have higher sensitivity to pollutants than structures designed in the visible region.
It is another purpose of the present invention to provide SPR sensors that possess appropriate stability for use in a water environment.
It is another purpose of the present invention to provide SPR sensors that overcomes the problem of detecting large biological entities.
Further purposes and advantages of this invention will appear as the description proceeds.